1. Field of the Invention
The present invention relates generally to an apparatus and method for transporting/receiving data in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a data transportation/reception apparatus and method for improving reliability of transport data bits.
2. Description of the Related Art
In a communication system, it is actually impossible to receive a transported signal without any distortions or noises. In particular, when the signal is transported/received over a wireless network, the effects of the distortions or noises are more serious as compared with when the signal is transported/received over a wired network.
Therefore, many efforts have been made to minimize the effects of the distortions or noises. An error control coding technique has been proposed as a typical method of minimizing the effects of the distortions or noises. Codes used for the error control coding technique are classified into a memoryless code and a memory code. For example, the memoryless code includes a linear block code, while the memory code includes a convolutional code and a turbo code. A device for generating such codes is called a “channel encoder”, and its outputs can be divided into systematic bits and parity bits based on the error control coding technique. The turbo code is a code typically used for the error control coding technique that separates its outputs into the systematic bits and the parity bits. In addition to the turbo code, there exists a systematic convolutional code of the convolutional code, as a code used for the error control coding technique.
Herein, the “systematic bits” mean an actual transport signal, while the “parity bits” mean a supplemental signal added to correct a possible error which occurred during transportation in a decoding process. However, even though a signal is subjected to the error control coding, if a burst error occurs in the systematic bits or the parity bits, it is not easy to correct the burst error. Such a phenomenon frequently occurs while the signal passes through a fading channel, and an “interleaving” technique is typically used to prevent this phenomenon. The interleaving technique disperses a damaged part in several places rather than concentrating it on a single place, thereby complementing the error control coding technique.
Such interleaved signal is subject to mapping in a symbol unit in a digital modulator. The number of bits included in one symbol increases with the order of the modulator. Particularly, in the case of high-order modulation of over 16QAM (16-ary Quadrature Amplitude Modulation), one symbol includes information of over 4 bits, and the bits can be classified according to reliability. Here, the “reliability” can be represented by a probability that the bit values will be changed during transportation. For example, when one 16QAM-modualted symbol is subject to mapping on coordinates, information of leading two bits in the symbol has higher reliability since the leading two bits determine a quadrant of the coordinates, to which the symbol is to be mapped. This means that there is a lower probability that the information of the leading two bits will be changed during transportation. However, information of the remaining two bits in the symbol has lower reliability, since the remaining two bits determine one of the four regions obtained by dividing the determined quadrant. This means that there is a higher probability that the information of the remaining two bits will be changed during transportation. That is, of at least 3 bits included in one symbol, the bits determining a wider region have higher reliability and the bits determining a narrower region have lower reliability.
A transmitter of a common HSDPA (High-Speed Downlink Packet Access) radio communication system is comprised of a channel encoder, an interleaver and a modulator, as illustrated in FIG. 1.
Referring to FIG. 1, a tail bit generator 110 receives N transport blocks and adds associated tail bits to the respective transport blocks. A channel encoder 112 encodes the N tail bit-added transport blocks from the tail bit generator 110 and outputs coded symbols. The channel encoder 112 has at least one coding rate in order to encode the N transport blocks. The coding rate may be ½ or ¾. When the channel encoder 112 supports a plurality of coding rates through symbol puncturing or symbol repetition using a ⅙ or ⅕ mother encoder, an operation of selecting a coding rate from the available coding rates is required. In FIG. 1, the channel encoder 112 determines (selects) the coding rate under the control of a controller 120.
A rate matcher 114 rate-matches the coded symbols from the channel encoder 112. The rate matching is performed by repeating or puncturing the coded symbols, when a transport channel is subject to multiplexing or when the number of the output symbols of the channel encoder 112 is not identical to the number of symbols transported over a radio channel. An interleaver 116 interleaves the rate-matched coded symbols from the rate matcher 114. The interleaving is performed to minimize a possible data loss during transportation. An M-ary modulator 118 modulates the interleaved coded symbols by QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM or 64QAM modulation. The controller 120 controls an operation of the channel encoder (or turbo encoder) 112 and a modulation mode of the modulator 118 according to a current state of the radio channel. The HSDPA radio communication system uses AMCS (Adaptive Modulation and Coding Scheme) for the controller 120 in order to select the modulation modes (QPSK, 8PSK, 16QAM and 64QAM) according to the radio environment. Though not illustrated in the drawing, the CDMA mobile communication system spreads transport data using a Walsh code W and an orthogonal code PN, so that a corresponding mobile terminal (or UE (User Equipment)) can identify a channel and a base station (or Node B) transporting the data.
The structure of the transmitter has been described on the assumption that coded symbols are not divided into the systematic bits and the parity bits. However, the coded symbols output from the channel encoder 112 of the transmitter can be divided into the systematic bits and the parity bits. Of course, the systematic bits and the parity bits output from the channel encoder 112 have different priorities. Namely, in the case where errors occur in the transport data at a given rate, it is possible to perform relatively correct decoding when the errors occur in the parity bits rather than in the systematic bits. The reason is, as stated above, because the systematic bits are actual data bits while the parity bits are supplemental bits added to correct errors occurred during transportation in a decoding process.
However, the interleaver 116 of the conventional transmitter in the communication system performs interleaving regardless of the priorities of the systematic bits and the parity bits. That is, the conventional transmitter performs symbol mapping irrespective of the systematic bits and the parity bits.
Hence, the conventional radio communication system has a high error occurrence probability regardless of the priorities of the systematic bits and the parity bits, when transporting data over the wireless network. Accordingly, there is a need for a technique capable of reducing a probability that errors will occur in the systematic bits having higher priority than the parity bits, thereby improving system performance.